Referring to FIGS. 1A-B, a conventional low-voltage pad-mount distribution transformer 20 in a meter network. The transformer 20 includes a housing 21 that defines a wire-retaining void 23 configured to retain the terminal ends 25 of a plurality of output load wires 27. Typically, the load wires 27 are buried in the ground 31, and the terminal ends 25 extend up through the ground and into an open base 29 in the housing 21 that leads to the internal void 23. The transformer 20 is illustrated as a single-phase power distribution transformer, though it should be appreciated that the transformer could alternatively be provided as a polyphase transformer having, for instance, three phases. The housing 21 further includes a cover 33 that can be removed or opened so as to selectively provide access to the void 23 and the contained electrical components.
The transformer 20 includes a plurality of output terminal assemblies 32 mounted onto an inner wall of the housing 21. Each output terminal assembly 32 includes an electrically insulating bushing mounting flange 35, an electrically insulating bushing 22, an electrically conductive output stud 28, and an electrically conductive wiring terminal 30 that receives a plurality of the output load wires 27. It should be appreciated that the flange 35 can be integrally connected to the bushing 22, or can be discretely connected to the bushing 22. It should further be appreciated that the electrically conductive components of the terminal assemblies 32 are current carrying conductors, and thus also current carrying members. Furthermore, the insulative components of the terminal assemblies 32 that carry a conductor (such as the bushing 22) while not conductive themselves, are current carrying members that carry the current of the current carrying conductors.
In accordance with the illustrated embodiment, the panel mount 35 can be bolted onto, or otherwise attached to or supported by the housing 21. The insulated bushing 22 can be cylindrical or alternatively shaped, and projects out from the panel mount 35 into the wire-retaining void 23. The electrically conductive output stud 28 defines a proximal end extending through the bushing 22 and panel mount 35, and connects to a high voltage source. The output stud 28 further defines a distal end opposite the proximal end that extends into the conductive wiring terminal 30. An exposed portion 41 of the output shaft 28 thus extends through a gap 38 disposed between the bushing 22 and the wiring terminal 30.
The wiring terminal 30 includes a plurality of output stud locks 34 that can include a set screw 39 or other suitable structure sufficient to retain the output stud 28 in the terminal 30 and provide a secure electrical connection between the wiring terminal 30 and the output stud 28. The wiring terminal 30 further includes a plurality of wire mounting apertures 36 that receive the terminal ends 25 of the wires 27. The wiring terminal 30 further includes a plurality of clamps 37 that can include a set screw or other suitable structure sufficient to retain the terminal ends 25 in the wire mounting apertures 36, thereby providing a secure electrical connection between the wires 27 and the wiring terminal 30. Accordingly, the output stud 28 is placed in electrical communication with the output load wires 27, which provide electrical power to a plurality of local nodes. It is appreciated that the output terminal assembly 32 is illustrated in accordance with one embodiment, and that numerous alternative configurations are known. The embodiments described herein are intended to be used in combination with all such alternative embodiments.
It is desirable to measure the electrical power flowing through each output terminal assembly 32, for instance to reconcile the power usage as measured at the nodes with the power applied at the transformer. Because the output stud 28 carries high currents, for instance on the order of approximately 100 Amps of alternating current, it is recognized that it is desirable to provide high reliable contact and low resistance at the interface between the output stud 28 and the wiring terminal 30. As a result, the output stud 28 projects as deep into the wiring terminal 30, which minimizes the gap 38 and corresponding exposed portion 41 of the output shaft 28. For instance, the gap 38, and corresponding exposed portion 41, can be as small as 0.5 inch. Furthermore, the output stud 28 has a diameter or cross-sectional dimension less than that of the bushing 22 and wiring terminal 30. Accordingly, access to the exposed portion 41 of the output stud 28 is limited.
What desirable is a method and apparatus for sensing current of a terminal assembly of a pad-mount distribution transformer.